1. Technical Field
This invention relates generally to pistons for internal combustion engines, including steel pistons for diesel engines, and methods of manufacturing the pistons.
2. Related Art
Steel pistons of various designs are used in diesel combustion engines. To accommodate combustion pressures and thermal loads, diesel engines are bulky by design. The engine design parameters affecting internal and external proportions of the engine are combustion bore diameters, spacing between adjacent cylinders which is dictated by cylinder type, stroke, compression height of the pistons, and center-to-center length of connecting rods, among others. Typically, if all other design parameters are kept constant, pistons with high compression height values, i.e. a compression height/diameter ratio of about 0.8, are dynamically difficult to stabilize because a high percentage of the mass of the piston is above the center of pivoting, which is along a pin bore axis of the piston. During translation of the piston, i.e. the sideways motion of the piston inside the cylinder, the piston instability generates impacts on the cylinder liner, referred to as piston slap. The piston slap is perceived as airborne noise, which can be disturbing, especially during cold start conditions. Under engine running conditions, the piston slap can cause cavitation of coolant and thus collapsing bubbles, which can lead to erosion of the cylinder liner surface.
The connecting rod length can also contribute to the noise if the angularity of the rod is too high, for example due to a short connecting rod. However, if the connecting rod is longer, the negative effects are minimized, and friction by the thrust force generated is also reduced. However, a longer connecting rod inevitably is heavier and larger at its small end, which in turn may force the use of a piston having a higher compression height. Bulkiness could also result from the need to dampen impulse noise generated by diesel combustion, referred to as diesel knock. At and above 6 bar per crank angle degree pressure rise rate, airborne noise becomes very objectionable. The pressure rise results mostly from the pre-mixed fuel injected since the start of injection.